The substantial growth of the communications industry has highlighted the limited capacity of existent data communication systems to handle rapidly expanding applications. In order to increase the capacity of the systems, various methods have been considered, for example frequency division multiplexing (FDM) or time division multiplexing (TDM).
Communication networks, such as telephone networks, often multiplex numerous data channels onto a given data transmission medium. In one embodiment, a data channel is a voice channel associated with a given communication. Alternatively, data channels may include a dedicated computer modem channel, or a telephone network maintenance channel. In one embodiment, a data transmission medium is a wire transmission line. Alternatively, the data transmission medium may be a radio frequency bandwidth, a fiber optic line, or any other type of communication link.
TDM combines the data channels by assigning each channel a different time slot in a set of sequential time slots and by repeatedly transmitting a fixed sequence of time slots over the single data transmission medium. In one embodiment, each data transport unit or time slot includes 8 bits of data and the fixed sequence of time slots is 125 microseconds. As a result, each 8-bit data transport unit or time slot is transmitted every 125 microseconds.
TDM switches are a growing part of the existing voice networks and are increasingly found in voice-enabled networking equipment. The use of TDM switches in packet-based networking equipment requires flexibility in size and assignment of ports, and requires support for multiple switching domains, each switching domain having independent timing.